In a conventional way of interconnecting equipment, each equipment incorporates a transmitter-receiver unit and the transmitter-receiver units of all equipments are connected to a common bus, which may be duplicated.
Transmission by the various units on the bus is governed by a priority process, transmissions being sequenced according to the respective requirements of the equipments and within the overall framework of a given time-division transmission organization.
These organizations are defined, for example, in CCITT Recommendation G.704 which specifies various synchronous frame structures for transferring digital signals in the form of octets over a link operated on a timesharing basis. The bits constituting the signals transmitted in succession by the various transmitter units are transmitted serially by the links.
In a conventional way the structures referred to above are used to transmit octets embodying signals produced by conventional primary PCM multiplexers conforming to CCITT Recommendation G.732 for the transmission of speech signals on isochronous channels at the rate of one octet per channel and per frame following sampling and encoding of the speech signals in the form of octets.
An isochronous channel, for example, enables transmission of encoded speech signals relating to one of the two transmission directions of a communication between two telephones.
These structures also enable transmission of octets embodying information in digital form grouped into packets comprising a fixed number of octets which are transmitted in preferably successive frame time slots. An asynchronous channel is then formed by the succession of time slots used by a unit to send one or more consecutive cells of a packet.
It is known to transmit isochronous channel octets and cell octets in the same frames on the same link using time slots left free by the isochronous channels during consecutive frames to transmit the cells serially, octet by octet.
Single or duplicated link transmission systems of this type are very suitable if the digital bit rate is relatively low and the transmission is implemented electrically, the number and the respective bit rates of units connected being matched to the possibilities offered by the equipment chosen for the system and by the organization adopted.
To obtain the digital bit rates required by some systems it is known to interconnect the equipments by a transmission system whose bus is made up of "p" identical links in parallel. Each equipment is temporarily assigned all of the "p" links to send "p" data groups.
The links are preferably assigned to the equipments of a common system to meet their transmission requirements temporarily and on demand if said requirements vary during use; variations occur, for example, if the equipments are parts of a switching center in an integrated services digital network.
A known temporary assignment method consists in delegating the responsibility for assigning the bus to a master unit which may be incorporated into one of the equipments, which receives assignment requests and which is responsible for arbitrating any access conflicts. This is not entirely satisfactory from the security point of view in that to alleviate any failure of the master unit likely to block the transmission system it is standard practise to duplicate or even preferably to triplicate the master unit and the means enabling it to communicate with the other equipments. The solution then becomes complex, costly and increasingly unreliable.
Another solution is to have the equipments themselves solve problems of temporary assignment of the bus to the equipments requiring to transmit.
A contention structure common to all the interconnected equipments, for example, enables the exchange of specific contention information intended to allow the successive assignment of the bus to each of the equipments waiting to transmit, on the basis of requirements and constraints specific to those equipments, to the bus and, more generally, to the system as a whole.
A known contention structure provides for the addition of a supplementary contention link to which all the equipments of a transmission system are connected. Signalling is transmitted on this contention link by the equipment transmitting at a given time temporarily to prohibit transmission via the system by any other equipment, arrangements being made at the level of the organization of the equipment and, more generally, the level of the transmission system so that each equipment can normally transmit within time-delays compatible with its needs.
Failures affecting the contention link can block the transmission system and the solutions of the type mentioned above in relation to the first known solution succinctly outlined above lead to the same disadvantages which can be highly troublesome with a large number of transmission systems designed to operature virtually continuously and in particular when the system operates in real time, as in the case of the switching center mentioned above.